Nano-copper powder has advantages including small dimensions, large specific surface area, low resistance, quantum size effect, macroscopic quantum tunneling effect, etc., and has a very important application value in the field of metallic electrically conductive ink. Copper is lower in price when compared with silver, and can greatly reduce the cost. Especially, the research on preparation and application of copper powder, which is a potential substitute for precious metal powder, has received wide attention in the world.
Nano-copper preparation methods include physical methods and chemical methods. Physical methods include mechanical milling method and gamma ray method. Chemical methods include solution phase reduction method, micro-emulsion method, solvothermal method, vapor deposition method, electrolytic method, and plasma method, etc. The existing method for preparing nano-copper through a solution phase reduction process requires high temperature for reaction and demanding experiment conditions. CN101386723B discloses a method, which employs sodium hypophosphite as the reducer, cupric sulfate as the precursor, LD and PVP as the disperser, and diethylene glycol (DEG) as the organic phase to prepare nano-copper with a particle diameter of 20 nm to 50 nm at a temperature of 120° C. to 160° C. However, the nano-copper powder obtained with that method shows uneven particle diameter; moreover, the method has a low yield ratio, and requires a high temperature in the presence of organic solvent for protection.
A method that utilizes metal borohydride as the reducer and obtains nano-copper by reducing copper salt from strong alkaline solution with a pH value of higher than 12 at a temperature of 90° C. to 160° C. has been widely reported in the world. M. Yu. Koroleva, D. A. Kovalenko, V. M. Shkinev et at (Russian Journal of Inorganic Chemistry, 2011, 56(1): 6-10) prepared spherical copper nano-particles with a particle diameter of 25 nm to 35 nm by reducing the water solution of Cu(NO3)2 with NaBH4 in the presence of polyoxyethylene sorbitan monooleate as disperser. However, when that method is used to prepare nano-copper, the reaction is vehement and the reaction system is unstable; in addition, the obtained copper powder product tends to agglomerate.
At present, nano-copper electrically conductive ink products existing in the market are only dispersible in water or alkanes (e.g., n-hexane, tetradecane, etc.); therefore, only water-based electrically conductive ink products or solvent-type electrically conductive ink products can be obtained. Since the principal component in water-based electrically conductive ink is water, leading to a low volatilization rate, and thus, circuits printed by water-based electrically conductive ink are not easy to dry. Consequently, the medium as support should have special coating; electronic circuits prepared with water-based electrically conductive ink show poor weather resistance, and it is difficult to maintain long-term performance stability of such electronic circuits in humid environments. The worst drawback of solvent-type electrically conductive ink is severe environmental pollution, since the volatile organic content in the ink is very high. In consideration of environmental protection, the application of solvent-type electrically conductive ink will be restricted gradually.
Hence, it is of great significance to provide nano-copper powder that is dispersible in water and environment-friendly weak solvents for the development of weak solvent-type electrically conductive ink.